There are a number of filters or facets in the left sidebar (highlighted in green) of the search results screen that allow you to further refine your search. Some of these filters are highlighted by section in yellow in the screen shot below (others not illustrated here include Language and Subject Terms):
In the CONTENT TYPE area, a few filters are listed, but the "More...." link at the bottom of that section leads to another panel of even more types of content to choose from.
To activate these limits, click on the option(s), and a green check mark will appear beside each active option.
Some of the most popular filters are:
Remember that if you don't see the kind of article or other content type that you want, click on the "More..." button in the left sidebar, and it will bring up another panel that gives you more options. To apply an option, click on the green check mark next to it (in this example, Magazines, circled in aqua); it will make "Magazines" appear in green also, and then scroll to the top of the box to click the "Apply" button (circled in orange) to make that filter stick: